Ultradense, 3-D Data Storage

Chips that use graphite show promise for storing more bits than flash memory.

For decades, engineers have tweaked chip design to store more data in a smaller space. But as chip components continue to shrink, engineers are looking at alternatives to silicon that might provide better performance at small sizes. One possible approach is to use carbon in the form of nanotubes--tiny, rolled-up sheets of carbon atoms--or in the form of graphene--single, flat sheets of the atoms. However, neither of these structures is easy to mass-produce and to integrate onto chips using existing manufacturing processes.

Cracking memory: These scanning electron microscope images of the graphite strip show an unaltered memory cell (top) and a cell that holds a bit of data (bottom), as represented by the crack. Credit: James Tour

But now, researchers at Rice University in Houston have shown that graphene's cousin, graphite, can be used to make a fast, high-density memory device with some of the advantages of flash memory typically found in memory cards and MP3 players. Graphite, the same material found in pencils, comes in multiple sheets and flakes, and can be deposited onto silicon using standard deposition processes, unlike nanotubes and graphene.

The graphite memory device, built by James Tour, professor of chemistry at Rice University, and postdoctoral researcher Alexander Sinitskii, is similar to flash in that it has no moving parts, which means it's more robust than a magnetic hard drive. But unlike flash memory, which stores bits as electrical charge, graphitic memory won't wear out as quickly. And graphite memory cells can be vertically aligned and stacked, which means that a chip using graphite has the potential to store 10 times more bits in the same space than today's flash memory.

A graphite memory cell is composed of sheets of graphite deposited between two electrodes. The two-electrode design of graphitic memory differs from that of flash memory, which requires a "source," a "drain," and a "gate" to hold electric charge--essentially the bits of data. Because flash memory must store charge on the gates, which tend to leak, the cells wear out over time.

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Graphitic memory works differently. When a certain voltage is applied to a memory cell, the strip of graphite cracks, explains Tour. The presence or absence of a crack--represented as a 0 or a 1--can be read by applying a lower voltage across the electrodes. Applying a larger voltage smoothes the crack, essentially erasing the bit. Tour admits that he isn't sure of the exact mechanism that occurs during the process of writing data, but he suspects that the voltage creates a filamentary structure within the carbon that interacts with the surrounding silicon, producing a characteristic electrical signature.

The two-electrode structure of graphitic memory is what enables it to be built in a three-dimensional memory cell, explains Tour. The three-component structure of flash memory makes it overly complicated to connect memory cells vertically. Graphitic memory, on the other hand, can easily be deposited between two layers of electrodes.